1. Field of the Invention
The present invention relates to a television signal and FM broadcast signal demodulator and a television signal and FM broadcast signal receiver.
2. Description of the Prior Art
A prior-art television signal and FM broadcast signal receiver will be described with reference to FIG. 6. Referring to FIG. 6, a VHF band television signal or an FM broadcast signal is inputted from an input terminal 31 of a receiver to a VHF tuner unit 32. The VHF tuner unit 32 includes a tuner, a mixer, a local oscillator or the like, though not shown, to select a received television signal or FM broadcast signal from the inputted television signal or FM broadcast signal and in which the mixer and the local oscillator frequency-convert a selected television signal or FM broadcast signal to provide a television intermediate frequency signal IF. Here, the intermediate frequency signal IF is frequency-converted in such a manner that a television video intermediate frequency signal VIF may have a frequency of 58.75 MHz. A frequency of a television audio intermediate frequency signal SIFtv becomes 54.25 MHz accordingly. Also, an FM audio intermediate frequency signal SIFfm is frequency-converted in such a manner that its frequency becomes 54.25 MHz which is the same frequency as that of this television audio intermediate frequency SIFtv.
Then, when a television signal is received, the video intermediate frequency signal VIF and the audio intermediate frequency signal SIFtv are inputted through an intermediate frequency circuit 33 to a demodulator 34. The demodulator 34 demodulates the video intermediate frequency signal VIF and the audio intermediate frequency signal SIFtv to provide a video signal V and an audio signal Atv. Also, when an FM broadcast signal is received, a second mixer 35 and a second local oscillator 36 frequency-convert an FM audio intermediate frequency signal SIFfm (54.25 MHz) outputted from a VHF tuner unit 32 to provide an audio intermediate frequency signal of 10.7 MHz, and an FM detector 37 outputs an audio signal Afm. The VHF tuner unit 32 is able to also receive an FM broadcast signal having a frequency lower than that of the television signal accordingly.
FIG. 7 is a block diagram of the demodulator 34 for demodulating the television signal. As shown in FIG. 7, the intermediate frequency signal IF from the intermediate frequency circuit 33 contains the video intermediate frequency signal VIF and the audio intermediate frequency signal SIFtv. These video and audio intermediate frequency signals VIF and SIFtv are inputted to a video intermediate frequency amplifier (simply referred to as xe2x80x9cVIFAMPxe2x80x9d) 41 and an audio intermediate frequency amplifier (simply referred to as xe2x80x9cSIFAMPxe2x80x9d) 42, respectively. Then, the video intermediate frequency signal VIF that was amplified by the VIFAMP 41 is inputted to a video detector 43 and an automatic phase controller (simply referred to as xe2x80x9cAPCxe2x80x9d) 44. The APC 44 phase-compares an oscillation signal from a voltage-controlled oscillator (simply referred to as xe2x80x9cVCOxe2x80x9d) 45 and the video intermediate frequency signal VIF. A compared voltage corresponding to a phase difference is inputted from the APC 44 to the VCO 45. As a consequence, the VCO 45 oscillates at 58.75 MHz which is a frequency of a video carrier of the video intermediate frequency signal VIF. Thus, the APC 44 and the VCO 45 comprise a PLL (phase-locked loop) circuit and serves as a video carrier generator to generate a video carrier. This video carrier is inputted to the video detector 43.
Then, the video detector 43 synchronously detects the video intermediate frequency signal VIF by the video carrier of 58.75 MHz to provide a video signal V.
The audio intermediate frequency signal SIFtv that was amplified by the SIFAMP 42 is inputted to an audio intermediate frequency detector 46, and the video carrier from the VCo 45 also is inputted to this audio intermediate frequency detector 46. Then, since an audio carrier of the audio intermediate frequency signal SIFtv is 54.25 MHz, the audio intermediate frequency detector 46 outputs a second audio intermediate frequency signal SIF2 of 4.5 MHz which is a difference of frequencies between the video carrier and the audio carrier. That is, the audio intermediate frequency detector 46 intercarrier-detects the audio intermediate frequency signal SIFtv to provide the second audio intermediate frequency signal SIF2.
Then, this second audio intermediate frequency signal SIF2 is inputted through a bandpass filter 47 having a center frequency of 4.5 MHz to an FM detector 48. This FM detector 48 outputs an audio signal Atv.
On the other hand, when an FM broadcast signal is received, since the intermediate frequency signal SIFfm based on the FM broadcast signal outputted from the intermediate frequency circuit 33 does not contain the video intermediate frequency signal VIF, the VCO 45 is unable to output the carrier of 58.75 MHz. Accordingly, the audio intermediate frequency detector 46 also is unable to output the second audio intermediate frequency signal SIF2 of 4.5 MHz. As a result, the demodulator 34 cannot FM-detect the FM broadcast signal.
In order to solve the above-mentioned problem, as mentioned before, after the second mixer 35 and the second local oscillator 36 have frequency-converted the audio intermediate frequency signal SIFfm based on the FM broadcast signal from the VHF tuner 32 to provide the audio intermediate frequency signal of 10.7 MHz, the FM detector 37 FM-detects this audio intermediate frequency signal.
As described above, according to the prior-art circuit arrangement, since the demodulator 34 for obtaining the television video signal V and audio signal Atv cannot FM-detect the FM broadcast signal, the second mixer 35 and the second local oscillator 36 are used to frequency-convert the FM broadcast signal to provide the audio intermediate frequency signal of 10.7 MHz, and then the audio intermediate frequency signal is FM-detected. For this reason, since the prior-art receiver required the second mixer 35, the second local oscillator 36 and the FM detector 37 extra, the circuit arrangement of the prior-art receiver becomes complicated, and the prior-art receiver becomes expensive unavoidably.
In view of the aforesaid aspect, it is an object of the present invention to provide a television signal and FM broadcast signal demodulator in which both of a television signal and an FM broadcast signal may be demodulated only by switching the reception of the television signal and the reception of the FM broadcast signal.
It is another object of the present invention to provide a television signal and FM broadcast signal receiver in which, when an FM broadcast signal is received, a common demodulator may obtain television video signal and audio signal and an audio signal of FM broadcast signal without using a second mixer for frequency-converting an audio intermediate frequency signal of 54.25 MHz to provide an audio intermediate frequency signal of 10.7 MHz.
According to a first aspect of the present invention, there is provided a television signal and FM broadcast signal demodulator which is comprised of a first input terminal to which a video intermediate frequency signal in a television intermediate frequency signal is inputted, a second input terminal to which an audio intermediate frequency signal in the intermediate frequency signal or an FM broadcast signal, which is frequency-converted so as to have the same frequency as that of the audio intermediate frequency signal, is inputted, a third input terminal to which a pseudo carrier having the same frequency as that of a video carrier in the intermediate frequency signal is inputted, a change-over switch for selecting either the video intermediate frequency signal inputted to the first input terminal or the pseudo carrier inputted to the third input terminal, a video detector to which one of the video intermediate frequency signal and the pseudo carrier selected by the change-over switch is inputted, an audio intermediate frequency detector to which the audio intermediate frequency signal or the FM broadcast signal inputted to the second input terminal is inputted and generating a second audio intermediate frequency signal, an FM detector for detecting the second audio intermediate frequency signal to provide an audio signal, and a video carrier generator to which the one signal is inputted, reproducing the video carrier and inputting the reproduced video carrier into the video detector and the audio intermediate frequency detector, wherein the change-over switch connects the first input terminal, the video detector and the video carrier generator to receive the television signal and the change-over switch connects the third input terminal, the video detector and the video carrier generator to receive the FM broadcast signal.
According to a second aspect of the present invention, there is provided a television signal and FM broadcast signal demodulator which is comprised of a first input terminal to which a video intermediate frequency signal in a television intermediate frequency signal is inputted, a second input terminal to which an audio intermediate frequency signal in the intermediate frequency signal and an FM broadcast signal, which is frequency-converted so as to have the same frequency as that of the intermediate frequency signal, are inputted, a third input terminal to which a pseudo carrier having the same frequency as that of a video carrier in the intermediate frequency signal is inputted, a video detector to which the video intermediate frequency signal inputted to the first input terminal is inputted and outputting a video signal, an audio intermediate frequency detector to which the audio intermediate frequency signal inputted to the second input terminal is inputted and generating a second audio intermediate frequency signal, an FM detector for detecting the second audio intermediate frequency signal to provide an audio signal, a video carrier generator to which said video intermediate frequency signal is inputted, reproducing the video carrier and inputting the reproduced video carrier into the video detector, and a change-over switch connected between the video carrier generator and the audio intermediate frequency detector, wherein the change-over switch connects the video carrier generator and the audio intermediate frequency detector to receive the television signal and the change-over switch connects the third input terminal and the audio intermediate frequency detector to receive said FM broadcast signal.
According to a third aspect of the present invention, there is provided a television signal and FM broadcast signal receiver which is comprised of a VHF tuner unit to which a television signal or an FM broadcast signal is inputted and frequency-converting the inputted television signal or FM broadcast signal to provide an intermediate frequency signal having a television intermediate frequency, a demodulating unit for demodulating the intermediate frequency signal to provide television video signal and audio signal, and a pseudo carrier generator for generating a pseudo carrier having the same frequency as that of a video carrier in the intermediate frequency signal, wherein the demodulating unit includes a video detector to which the video intermediate frequency signal in the intermediate frequency signal is inputted and outputting a video signal, a video carrier generator to which the video intermediate frequency signal is inputted and reproducing the video carrier based on the inputted video intermediate frequency signal, an audio intermediate frequency detector to which an audio intermediate frequency signal in the intermediate frequency signal and the video carrier reproduced by the video carrier generator are inputted and outputting a second audio intermediate frequency signal, and an FM detector for detecting the second audio intermediate frequency signal to provide an audio signal and in which when the FM broadcast signal is received, the pseudo carrier is inputted to the audio intermediate frequency detector instead of the video carrier based on the intermediate frequency signal.
In accordance with a fourth aspect of the present invention, there is provided a television signal and FM broadcast signal receiver which further comprises a change-over switch for selecting either the video intermediate frequency signal or the pseudo carrier and wherein the change-over switch inputs the video intermediate frequency signal into the video detector and the video carrier generator when the television signal is received, the change-over switch inputs the pseudo carrier into the video detector and the video carrier generator and the pseudo carrier signal from the video carrier generator is inputted to the audio intermediate frequency detector when the FM broadcast signal is received.
In accordance with a fifth aspect of the present invention, there is provided a television signal and FM broadcast signal receiver which further comprises a change-over switch connected between the video carrier generator and the audio intermediate frequency detector and wherein the change-over switch inputs the video carrier from the video carrier generator into the audio intermediate frequency detector when the television signal is received, and the change-over switch inputs the pseudo carrier from the pseudo carrier generator into the audio intermediate frequency detector when the FM broadcast signal is received.